U.S. patent application number 14/899579 was filed with the patent office on 2016-06-09 for process and apparatus for producing ammonium sulfate crystals.
The applicant listed for this patent is CAP III B.V.. Invention is credited to Geert Ekkelenkamp, Robert Geertman, Johan Thomas Tinge.
Application Number | 20160159657 14/899579 |
Document ID | / |
Family ID | 48699577 |
Filed Date | 2016-06-09 |
United States Patent
Application |
20160159657 |
Kind Code |
A1 |
Tinge; Johan Thomas ; et
al. |
June 9, 2016 |
PROCESS AND APPARATUS FOR PRODUCING AMMONIUM SULFATE CRYSTALS
Abstract
The present invention provides a continuous process for
producing ammonium sulfate crystals, wherein said process
comprises: (a) feeding to a first group of crystallization
sections, which crystallization sections are heat integrated in
series, a first aqueous ammonium sulfate solution that contains one
or more impurities; (b) feeding to a second group of
crystallization sections, which crystallization sections are heat
integrated in series, a second aqueous ammonium sulfate solution
that contains one or more impurities; (c) crystallizing ammonium
sulfate crystals in each crystallization section respectively from
each of said solutions of ammonium sulfate that contain one or more
impurities; (d) purging a fraction of the ammonium sulfate solution
that contains one or more impurities from each of said
crystallization sections; and (e) discharging ammonium sulfate
crystals from each crystallization section, characterized in that:
(i) both the first group of crystallization sections and the second
group of crystallization sections are together heat integrated in
one series of crystallization sections; wherein the first group of
crystallization sections operates at higher temperature than the
second group of crystallization sections; and (ii) the composition
of the first aqueous ammonium sulfate solution that contains one or
more impurities is different to the composition of the second
aqueous ammonium sulfate solution that contains one or more
impurities. Further provided is apparatus suitable for producing
ammonium sulfate crystals.
Inventors: |
Tinge; Johan Thomas;
(Sittard, NL) ; Ekkelenkamp; Geert; (Echt, NL)
; Geertman; Robert; (Echt, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CAP III B.V. |
Sittard |
|
NL |
|
|
Family ID: |
48699577 |
Appl. No.: |
14/899579 |
Filed: |
June 20, 2014 |
PCT Filed: |
June 20, 2014 |
PCT NO: |
PCT/EP2014/062987 |
371 Date: |
December 18, 2015 |
Current U.S.
Class: |
423/545 ;
422/245.1 |
Current CPC
Class: |
C07D 201/16 20130101;
B01D 9/0022 20130101; B01D 9/004 20130101; B01D 9/0063 20130101;
C07D 201/04 20130101; B01D 2009/0086 20130101; B01D 9/0031
20130101; C07C 249/08 20130101; C07C 2601/14 20170501; C01C 1/24
20130101; C07C 251/44 20130101; C01C 1/248 20130101; C07C 249/08
20130101 |
International
Class: |
C01C 1/24 20060101
C01C001/24; B01D 9/00 20060101 B01D009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 24, 2013 |
EP |
13173325.5 |
Claims
1. A continuous process for producing ammonium sulfate crystals,
wherein said process comprises: (a) feeding to a first group of
crystallization sections, which crystallization sections are heat
integrated in series, a first aqueous ammonium sulfate solution
that contains one or more impurities; (b) feeding to a second group
of crystallization sections, which crystallization sections are
heat integrated in series, a second aqueous ammonium sulfate
solution that contains one or more impurities; (c) crystallizing
ammonium sulfate crystals in each crystallization section
respectively from each of said solutions of ammonium sulfate that
contain one or more impurities; (d) purging a fraction of the
ammonium sulfate solution that contains one or more impurities from
each of said crystallization sections; and (e) discharging ammonium
sulfate crystals from each crystallization section, characterized
in that: (i) both the first group of crystallization sections and
the second group of crystallization sections are together heat
integrated in one series of crystallization sections; wherein the
first group of crystallization sections operates at higher
temperature than the second group of crystallization sections; and
(ii) the composition of the first aqueous ammonium sulfate solution
that contains one or more impurities is different to the
composition of the second aqueous ammonium sulfate solution that
contains one or more impurities.
2. A process according to claim 1, wherein the crystallization
sections are heat integrated by means of steam.
3. A process according to claim 2, wherein the temperature of steam
entering the first crystallization section in the series of
crystallization sections is from 80.degree. C. to 160.degree.
C.
4. A process according to claim 3, wherein the temperature of steam
exiting the last crystallization section in the series of
crystallization sections is from 45.degree. C. to 75.degree. C.
5. A process according to claim 1, wherein the first aqueous
ammonium sulfate solution that contains one or more impurities and
the second aqueous ammonium sulfate solution that contains one or
more impurities are each independently produced as a by-product
during the production of cyclohexanone oxime, caprolactam or
acrylonitrile.
6. A process according to claim 5, wherein the first aqueous
ammonium sulfate solution that contains one or more impurities is
produced as a by-product during the production of cyclohexanone
oxime by oximation of cyclohexanone with aqueous hydroxylammonium
sulfate.
7. A process according to claim 5, wherein the second aqueous
ammonium sulfate solution that contains one or more impurities is
produced as a by-product during the production of caprolactam
obtained by Beckmann rearrangement of cyclohexanone oxime in either
oleum, sulfuric acid, or SO.sub.3.
8. A process according to claim 1, wherein the series of
crystallization sections comprises from 2 to 5 crystallization
sections.
9. A process according to claim 1, wherein each crystallization
section in the series of crystallization sections has substantially
equal production capacity of ammonium sulfate crystals.
10. A process according to claim 1, wherein a fraction of aqueous
ammonium sulfate solution that also contains one or more impurities
is purged from at least one crystallization section in a group to
at least one other crystallization section in the same group.
11. A process according to claim 10, wherein the crystallization
sections are heat integrated by means of steam, and a fraction of
aqueous ammonium sulfate solution that also contains one or more
impurities is purged from each crystallization section in a group
to the next crystallization section, as defined by descending
temperature of steam supply, in the same group, with the exception
that the purge from the final crystallization section in the group
is discharged from the group.
12. Apparatus suitable for producing ammonium sulfate crystals,
said apparatus comprising: (i) a first series of crystallization
sections, comprising: (a) a plurality of crystallization sections,
each crystallization section comprising a purge outlet; (b) a first
material feed system connected to each crystallization section in
the first series; (c) a first product removal system connected to
each crystallization section in the first series; (d) a first steam
supply system integrating the crystallization sections in series;
(ii) a second series of crystallization sections, comprising: (a) a
plurality of crystallization sections, each crystallization section
comprising a purge outlet; (b) a second material feed system
connected to each crystallization section in the second series; (c)
a second product removal system connected to each crystallization
section in the second series; (d) a second steam supply system
integrating the crystallization sections in series; characterized
in that the first steam supply system is connected to the second
steam supply system and the first material feed system is not
connected to the second material feed system.
13. Apparatus according to claim 12, wherein each of the
crystallizers in the first series has substantially equal
production capacity to the other crystallizers in the first series,
and each of the crystallizers in the second series has
substantially equal production capacity to the other crystallizers
in the second series.
14. Apparatus according to claim 13, wherein the production
capacity of ammonium sulfate crystals of each crystallization
section is at least 10,000 tons per annum.
Description
[0001] The invention relates to a process for preparing ammonium
sulfate crystals.
[0002] Ammonium sulfate ((NH.sub.4).sub.2SO.sub.4) is a product
which is produced on a large scale. This inorganic salt has a
number of commercial uses, but is used mainly as fertilizer in
agriculture to provide nitrogen and sulfur. It contains 21%
nitrogen as ammonium cations, and 24% sulfur as sulfate anions.
Ammonium sulfate crystals for this use are classified according to
crystal size.
[0003] In almost all commercial caprolactam production processes
ammonium sulfate is obtained as by-product. The ammonium sulfate
might be produced during the formation of the intermediate
cyclohexanone oxime and/or during the Beckmann rearrangement of
cyclohexanone oxime into caprolactam.
[0004] Hydroxylamine sulfate may be produced by the so-called
Raschig processes (including conventional Raschig process and
Direct Raschig process), using ammonium salts as starting
materials. Oximation of cyclohexanone is then carried out with
aqueous hydroxylammonium sulfate solution. In general ammonia is
added to liberate the hydroxylammonium, whereby ammonium sulfate is
formed. The amount of ammonium sulfate formed during oximation is
typically about 2.7 tons per ton of cyclohexanone oxime.
[0005] Hydroxylammonium sulfate solution may alternatively be
obtained by hydrogenation of nitric oxide over a platinum catalyst
in the presence of dilute sulfuric acid. The hydroxylammonium
sulfate solution is reacted with cyclohexanone and ammonia to form
cyclohexanone oxime and ammonium sulfate. This process for the
production of hydroxylammonium sulfate solution typically generates
about 0.8 tons of ammonium sulfate per ton of cyclohexanone oxime
during the oximation step.
[0006] In the Beckmann rearrangement reaction of cyclohexanone
oxime, either sulfuric acid or oleum or SO.sub.3 is used as
rearrangement medium. The reaction gives the sulfate of caprolactam
in excess sulfuric acid, which is then neutralized with ammonia or
ammonia water. This process for the production of caprolactam from
cyclohexanone oxime typically generates in the range of from 1.4 to
1.8 tons of ammonium sulfate per ton of converted cyclohexanone
oxime.
[0007] Most industrial acrylonitrile is produced by catalytic
ammoximation of propene. Ammonia is separated from the product
stream using sulfuric acid, resulting in an ammonium sulfate
solution. This process for the production of acrylonitrile from
propene typically generates about 0.2 tons of ammonium sulfate per
ton of produced acrylonitrile.
[0008] Ammonium sulfate crystals may be obtained by subjecting an
ammonium sulfate solution to crystallization and subjecting the
resulting slurry of ammonium sulfate crystals to a size
classification step.
[0009] In general, ammonium sulfate crystals are obtained, in a
crystallization step, from the ammonium sulfate solution by
evaporating the solvent, which solvent usually is water. Examples
of crystallizers are described in "Perry's Chemical Engineers
Handbook" by Don W. Green and James O. Maloney, 7th edition, McGraw
Hill, 1997, Section 18, pages 44-55. The temperature and pressure
at which the crystallizer is operated are not critical. However,
the crystallizer usually is operated at a temperature of between 20
and 180.degree. C. and at a pressure of between 2 kPa and 0.8
MPa.
[0010] Crystallization by evaporation typically involves heat input
to evaporate solvent and concentrate the remaining solution. In
order to reduce steam consumption needed for evaporative
crystallization in the production of sodium chloride crystals from
aqueous sodium chloride solutions, for example, a series of
crystallizers are in general integrated with respect to heat input
(see e.g. I. Kristjansson, Geothermics, 21 (1992); pp 765-771). In
a series of heat integrated crystallization sections, water is
boiled in a sequence of crystallizers, each held at a lower
pressure than the last. Because the boiling temperature of water
decreases as pressure decreases, the vapor boiled off in one
crystallizer can be used to heat the next, and only the first
crystallizer (at the highest pressure) requires an external source
of heat. This is commonly done by passing steam at a high
temperature into the reboiler of the first crystallizer in a
series. The resulting lower temperature steam is used to heat the
next crystallizer, and so on. This means that a series of
crystallizers operates at descending temperatures. The size and
conditions of the crystallizers are optimized for this heating
arrangement.
[0011] Unlike single-stage evaporative crystallizers, a series of
heat integrated crystallization sections can be made of up to seven
evaporator stages or effects. The energy consumption for a
single-effect evaporative crystallizer is very high and makes up
most of the variable cost for an evaporation system. Putting
together evaporators saves heat and thus requires less energy.
Adding one evaporator to the original decreases the energy
consumption to 50% of the original amount. Adding another effect
reduces it to 33% and so on. In practice the realized savings are
somewhat less, amongst other due to energy required for preheating
of the feeds to boiling temperatures.
[0012] US201110038781A1 describes a process and apparatus for the
crystallization of ammonium sulfate, comprising a precrystallizer
which provides heating from vapour of reaction in series to three
crystallizers. The crystallizers are operated in parallel with
respect to ammonium sulfate production from a single source.
[0013] Impurities present in aqueous ammonium sulfate solutions
obtained as by-product during the production of, for example,
cyclohexanone oxime, caprolactam or acrylonitrile, tend to form
visible solid impurities under the conditions of evaporative
crystallization. The presence of such visible solid impurities has
a negative impact on the quality of the produced ammonium sulfate
crystals. Formation of visible solid impurities is more pronounced
during crystallization at higher temperatures. Accordingly, it is
desirable not to crystallize the ammonium sulfate solution at
higher temperatures. Because heating of a series of crystallizers
is by multiple effect, avoiding crystallizers that are operated at
high temperatures limits the number of crystallizers possible in
the series. Therefore optimum use of available heat is not made.
Heat consumption of a series of crystallizers is accordingly higher
than otherwise possible.
[0014] During conventional operation of an evaporative
crystallizer, impurities become concentrated in the solution.
Accordingly, a purge is employed, whereby solution is, continuously
or periodically, discharged from the crystallizer. In a
conventional series of crystallization sections, each
crystallization section operates at the same concentration of
impurity. The purge therefore reduces the amount of visible solid
impurities in the ammonium sulfate crystals. However, the purge
does not adequately prevent formation of visible solid impurities.
Further, purging also removes ammonium sulfate solution from the
crystallization section, thereby reducing the yield of ammonium
sulfate crystals from solution. This has a negative impact on the
economics of ammonium sulfate crystal production.
[0015] The present inventors have recognised that the formation of
visible solid impurities is dependent on the impurities in the
ammonium sulfate solution. The temperature at which a given
ammonium sulfate solution may be crystallized without formation of
visible solid impurities is dependent on both the quantity and the
composition of its impurities. Accordingly, where two or more
sources of ammonium sulfate having different compositions of
impurity are to be crystallized, different upper temperatures may
be used for each source. Rather than combining such sources and
crystallizing them commonly, separate crystallizations at different
temperatures may be used for each source. Further it was found that
the crystallizations could be carried out separately, but
integrated with respect to heat supply in one series of
crystallization sections.
[0016] Accordingly, the present invention provides a continuous
process for producing ammonium sulfate crystals, wherein said
process comprises:
[0017] (a) feeding to a first group of crystallization sections,
which crystallization sections are heat integrated in series, a
first aqueous ammonium sulfate solution that contains one or more
impurities;
[0018] (b) feeding to a second group of crystallization sections,
which crystallization sections are heat integrated in series, a
second aqueous ammonium sulfate solution that contains one or more
impurities;
[0019] (c) crystallizing ammonium sulfate crystals in each
crystallization section respectively from each of said solutions of
ammonium sulfate that contain one or more impurities;
[0020] (d) purging a fraction of the ammonium sulfate solution that
contains one or more impurities from each of said crystallization
sections; and
[0021] (e) discharging ammonium sulfate crystals from each
crystallization section,
[0022] characterized in that:
[0023] (i) both the first group of crystallization sections and the
second group of crystallization sections are together heat
integrated in one series of crystallization sections; wherein the
first group of crystallization sections operates at higher
temperature than the second group of crystallization sections;
and
[0024] (ii) the composition of the first aqueous ammonium sulfate
solution that contains one or more impurities is different to the
composition of the second aqueous ammonium sulfate solution that
contains one or more impurities.
[0025] The present invention further provides apparatus suitable
for producing ammonium sulfate crystals, said apparatus
comprising:
[0026] (i) a first series of crystallization sections,
comprising:
[0027] (a) a plurality of crystallization sections, each
crystallization section comprising a purge outlet;
[0028] (b) a first material feed system connected to each
crystallization section in the first series;
[0029] (c) a first product removal system connected to each
crystallization section in the first series;
[0030] (d) a first steam supply system integrating the
crystallization sections in series;
[0031] (ii) a second series of crystallization sections,
comprising:
[0032] (a) a plurality of crystallization sections, each
crystallization section comprising a purge outlet;
[0033] (b) a second material feed system connected to each
crystallization section in the second series;
[0034] (c) a second product removal system connected to each
crystallization section in the second series;
[0035] (d) a second steam supply system integrating the
crystallization sections in series;
[0036] characterized in that the first steam supply system is
connected to the second steam supply system and the first material
feed system is not connected to the second material feed
system.
[0037] As used herein, a crystallization section comprises all
equipment necessary to accept an ammonium sulfate solution, and
discharge ammonium sulfate crystals. In its simplest form this
means a crystallizer and a separation unit.
[0038] A group of crystallization sections, which crystallization
sections are heat integrated in series means that heat is applied
to the crystallization section at one end of the group; heat is
then transferred, directly or indirectly, to the next
crystallization section in the group; and so on until the last
crystallization section in the group. In this way, a single
external heat source is used to heat all crystallization sections
in the group, but is applied directly only to the first.
[0039] By purging a fraction of the ammonium sulfate solution is
meant that a fraction of the ammonium sulfate solution is
discharged from the crystallization section. The purpose of the
purge is to reduce the impurity content in the ammonium sulfate
solution, thereby also reducing the impurity content of the
crystallized ammonium sulfate.
[0040] An ammonium sulfate solution consists just of solvent,
impurities and (pure) ammonium sulfate. The composition of the
first aqueous ammonium sulfate solution is different to the
composition of the second aqueous ammonium sulfate solution in at
least one of concentration of ammonium sulfate and concentration
and nature of impurities. Impurities may be organic or inorganic
impurities or both. The compositions of the impurities are
typically different in the first and second feed.
[0041] Ammonium sulfate crystals consist, besides some remaining
solvent, just of impurities and (pure) ammonium sulfate.
[0042] Together heat integrated in one series means that the two
groups of crystallization sections form one series. A series of
crystallization sections, which crystallization sections are heat
integrated in series means that heat is applied to the
crystallization section at one end of the series; heat is then
transferred, directly or indirectly, to the next crystallization
section in the series; and so on until the last crystallization
section in the series. In this way, a single external heat source
is used to heat all crystallization sections in the series, but is
applied directly only to the first. The solutions of ammonium
sulfate that contain one or more impurities fed to each group of
crystallization sections may be mixed, but preferably remain
independent. Accordingly, there is preferably no purge from one
group of crystallization sections to another group of
crystallization sections. Optionally, a purge from a
crystallization section might be fed to another crystallization
section.
[0043] The first group of crystallization sections operates at a
higher temperature than the second group of crystallization
sections. Therefore the first group is positioned earlier in the
heat integration series than the second group.
[0044] The apparatus of the present invention comprises
crystallization sections which are configured to be heat integrated
with respect to steam. By this it is meant that steam is the heat
source, and that the series of crystallization sections comprises
the necessary pipework that heat can be transferred by input of
steam to the first crystallization section in the series, and the
heat transferred throughout the series, as described above. The
temperature of successive crystallization sections in the series is
therefore successively lower. Therefore, there is a direction of
descending temperature of steam supply, from the first
crystallization section to the last crystallization section in the
series.
[0045] Crystals of a larger size are preferred, because they
generally have a larger economic value. Typically, the mean median
diameter of crystals produced is greater than 0.8 mm. Preferably,
the mean median diameter of the ammonium sulfate crystals
discharged is from 1.0 mm to 4.0 mm.
[0046] The formation of visible solid impurities is reduced by
employing the method and apparatus of the present invention.
Visible solid impurities comprise dark coloured solid impurities.
These are, in particular solid impurities with a high, and even
majority, organic content. Typical impurities arise from the method
by which the ammonium sulfate solution is produced. The nature of
impurities is typically different in the two different feeds of
aqueous ammonium sulfate solution.
[0047] Preferably, the ammonium sulfate solution is produced from a
process for producing .epsilon.-caprolactam or acrylonitrile.
Accordingly, the impurities present are typically those commonly
produced in such reactions and/or already available in its raw
materials. A blend of impurities from different sources could
require typically high mother liquor purging rates. For example,
the blend of nitrate impurity, typical from cyclohexanone oxime
synthesis of hydroxylamine sulfate; together with impurities having
high chemical oxygen demand (COD), typical from the Beckmann
rearrangement of cyclohexanone oxime is, especially after removing
of solvent, potentially explosive. Processing two aqueous ammonium
sulfate feeds independently avoids such a blend being formed.
[0048] In addition this method allows a reduced overall purge of
ammonium sulfate comprising mother liquor resulting in the
production of an increased fraction of pure ammonium sulfate
crystals.
[0049] Heat integration may be achieved by any suitable means. For
example, by vapour recompression or by multiple effect evaporation
(also called evaporation in effect). Preferably the crystallization
sections are heat integrated by means of multiple effect
evaporation. Various heat sources can be used as energy source for
the evaporation of solvent in the first evaporator of a series of
evaporators. Preferably steam is used as heat source for the
evaporation of solvent in the first evaporator of a series of
evaporators. Accordingly, preferably the crystallization sections
are heat integrated by means of steam. Steam is readily available
as a heat source on chemical production facilities.
[0050] A particular advantage of the present invention is that two
or more sources of ammonium sulfate having different impurity
profiles may be used. This allows efficient processing of ammonium
sulfate produced as by-product from two or more different
processes. Typically the first aqueous ammonium sulfate solution
that contains one or more impurities and the second aqueous
ammonium sulfate solution that contains one or more impurities are
each produced as a by-product during the production of another
chemical product. Preferably, the first aqueous ammonium sulfate
solution that contains one or more impurities and the second
aqueous ammonium sulfate solution that contains one or more
impurities are each produced as a by-product during the production
of cyclohexanone oxime, caprolactam and/or acrylonitrile.
[0051] Typically the first aqueous ammonium sulfate solution that
contains one or more impurities is produced as a by-product during
the production of cyclohexanone oxime. Preferably, it is obtained
as a by-product during the production of cyclohexanone oxime via
oximation of cyclohexanone with aqueous hydroxylammonium
sulfate.
[0052] Typically the second aqueous ammonium sulfate solution that
contains one or more impurities is produced as a by-product during
the production of caprolactam. Preferably, it is produced as a
by-product during the production of caprolactam obtained by
Beckmann rearrangement of cyclohexanone oxime in oleum, sulfuric
acid, or SO.sub.3.
[0053] Typically the first aqueous ammonium sulfate solution that
also contains one or more impurities and the second aqueous
ammonium sulfate solution that also contains one or more impurities
are each produced as by-products during the production of
cyclohexanone oxime obtained via oximation of cyclohexanone with
aqueous hydroxylammonium sulfate, caprolactam obtained by Beckmann
rearrangement of cyclohexanone oxime in oleum, sulfuric acid, or
SO.sub.3 and/or acrylonitrile obtained by ammoximation of
propene.
[0054] In principle, any number of crystallization sections may be
used in a group of crystallization sections. However, typically, a
group of crystallization sections comprises from 1 to 4
crystallization sections. Preferably, it comprises 2 or 3
crystallization sections. A series of crystallization sections
typically comprises from 2 to 8 crystallization sections.
Preferably, the series of crystallization sections comprises from 2
to 5 crystallization sections.
[0055] Typically the temperature of steam entering the first
crystallization section in the series of crystallization sections
is from 80.degree. C. to 160.degree. C. Preferably, it is from
100.degree. C. to 140.degree. C., for example 120.degree. C.
[0056] Typically the temperature of steam exiting the last
crystallization section in the series of crystallization sections
is from 40.degree. C. to 90.degree. C. Preferably, it is from
45.degree. C. to 75.degree. C., more preferably, it is from
45.degree. C. to 55.degree. C., for example 50.degree. C.
[0057] Typically, the ammonium sulfate is crystallized from the
aqueous phase under acid conditions. Preferably ammonium sulfate is
crystallized from the aqueous phase at a pH of from 2 to 6; more
preferably from 4 to 5 (as determined at a temperature of
25.degree. C.).
[0058] Typically, each crystallization section in the series of
crystallization sections has substantially equal production
capacity of ammonium sulfate crystals. As used herein, the term
substantially equal production capacity of ammonium sulfate
crystals means that production capacity typically deviates by less
than 10% between crystallization sections. Preferably, it is less
than 5%; more preferably less than 2%. Production capacity is
typically measured as mass of product produced in unit time. For
example, in kg per hour, or kilotons per annum (kta).
[0059] Typically a fraction of aqueous ammonium sulfate solution
that also contains one or more impurities is purged from at least
one crystallization section in a group to at least one other
crystallization section in the same group.
[0060] The purity of the majority of crystals produced by a group
of crystallization sections may be increased. By purging from one
crystallization section in the group to another crystallization
section in the group, the mean concentration of impurities in the
group of crystallization sections can be reduced. Further, the
system of purging can be arranged such that the purity of product
crystals from all but one of the crystallization sections in the
group is far higher than that of the prior art. Accordingly, the
produced crystals can be further processed as a particularly pure
product and a less pure product. Or the products can be combined to
produce a product which is on average more pure than a system
without purge coupling.
[0061] Typically the crystallization sections are heat integrated
by means of steam, and a fraction of aqueous ammonium sulfate
solution that also contains one or more impurities is purged from
each crystallization section in a group to the next crystallization
section, as defined by descending temperature of steam supply, in
the same group, with the exception that the purge from the final
crystallization section in the group is discharged from the group.
In such a way impurity build-up to levels that are undesired in
each of the crystallization sections is avoided.
[0062] Typically, each crystallization section comprises an
evaporative crystallizer and solid-liquid separation equipment. The
crystallizer may be of any suitable type. However, preferably, each
crystallization section comprises an Oslo-type crystallizer.
Oslo-type crystallizers are particularly suitable for the present
invention because they are capable of producing crystals of larger
mean median diameter. That is in general desirable for ammonium
sulfate crystals.
[0063] Solid-liquid separation equipment means any equipment
suitable to separate ammonium sulfate crystals from a solution
comprising ammonium sulfate. It may include a continuous filter, a
centrifuge, a decanter, an elutriation column, a hydrocyclone, a
salt leg or a combination thereof. For example, it may comprise a
salt leg in combination with an elutriation column and a
centrifuge. Typically, before leaving the crystallization section
the ammonium sulfate crystals are washed with water or an aqueous
ammonium sulfate solution. Typically, after leaving the
crystallization section the resulting ammonium sulfate crystals are
discharged to a drying section. Crystals from each crystallization
section may be combined either before or after drying.
[0064] In the apparatus of the present invention, the steam supply
system includes an heat integration of the first steam supply
system and the second steam supply system. In this way the steam
supply system may be a single system heat integrating both the
first series of crystallization sections and the second series of
crystallization sections in series. The material feed system may
feed a solution or a slurry. Preferably it is for a solution of
ammonium sulphate or a slurry of ammonium sulphate. Preferably, the
first product removal system is connected to the second product
removal system.
[0065] In a preferred embodiment, the apparatus of the present
invention is suitable for producing ammonium sulfate crystals, said
apparatus comprising:
[0066] (a) a series of crystallization sections, which are
configured to be heat integrated with respect to steam;
[0067] (b) a steam supply system integrating the crystallization
sections in series;
[0068] (c) a feed system configured to provide aqueous ammonium
sulfate solution that contains one or more impurities to the series
of crystallization sections;
[0069] (d) a purge system from each crystallization section;
and
[0070] (e) a system of removal of ammonium sulfate crystals from
each crystallization section;
[0071] characterized in that,
[0072] (i) the series of crystallization sections comprises a first
group of crystallization sections, and a second group of
crystallization sections wherein the first group of crystallization
sections is configured to receive steam of a higher temperature
than the steam supplied to the second group of crystallization
sections; and
[0073] (ii) the feed system comprises a first feed configured to
provide a first aqueous ammonium sulfate solution that contains one
or more impurities to the first group of crystallization sections;
and a second feed configured to provide a second aqueous ammonium
sulfate solution that contains one or more impurities to the second
group of crystallization sections.
[0074] The apparatus of the present invention is typically an
ammonium sulfate crystallization plant. Typically, such a plant is
integrated with one or more other chemical plants. For example a
plant for the production of caprolactam and/or cyclohexanone oxime
and/or acrylonitrile. Capacity of the ammonium sulfate
crystallization plant is typically selected based on the volume of
ammonium sulfate solution discharged from the other chemical
plants. Further, crystallization section size is selected based on
optimum conditions. Typically, the production capacity of ammonium
sulfate crystals of each crystallization section is on the scale of
thousands of tons per year (kilotons per annum; kta). Typically the
production capacity of ammonium sulfate crystals of each
crystallization section is more than 10,000 tons per annum (10
kta). Preferably, it is from 10 kta to 200 kta. More preferably,
the production capacity of each crystallization section is from 30
kta to 150 kta.
[0075] For a series of crystallization sections, it is preferred
that the crystallization sections are each of the same size and
type, because of lower investment costs. The production capacity is
important, because the steam used from one effect is used for the
next effect, as described in Kristjansson in Geothermics 21 (1992)
765-771. A further advantage of having substantially equal
production capacity is that equipment can be standardized. The
crystallizer and the separation units, for example centrifuges and
filters, are preferably respectively each of the same type.
Typically each of the crystallization sections in the series has
substantially equal production capacity to the other
crystallization sections in the series. In other words each of the
crystallizers in the first series has substantially equal
production capacity to the other crystallizers in the first series,
and each of the crystallizers in the second series has
substantially equal production capacity to the other crystallizers
in the second series.
[0076] The present invention will be more fully explained with
reference to the following drawings.
[0077] FIG. 1 describes an embodiment of the prior art, wherein
four crystallization sections are arranged in parallel in view of
the feed of ammonium sulfate solution. FIG. 2 describes an
embodiment of the present invention, wherein the common feed line
is adapted to enable feeding solutions of ammonium sulfate with
different compositions to two groups of two crystallization
sections. FIG. 3 describes an embodiment of the present invention
comprising two parallel series, each of four crystallization
sections, each series heat integrated in effect. Two feed ammonium
sulfate solutions are fed to four groups of crystallization
sections across the two series.
[0078] FIG. 1 describes an embodiment of the prior art. Four
crystallization sections, (1), (2), (3), (4), each comprising a
crystallizer of equal size are arranged in parallel with respect to
the feed of ammonium sulfate solution. An ammonium sulfate solution
passes through feed line (5) into each crystallization section,
where crystallization occurs to form a slurry of ammonium sulfate
crystals in an ammonium sulfate solution. The ammonium sulfate
solution that passes through feed line (5) might originate from one
single source or might have been obtained by blending two or more
solutions of ammonium sulfate originating from different
sources.
[0079] Steam is fed to the crystallization section (1), via line
(6), where it is used to evaporate solvent from the ammonium
sulfate solution, thereby aiding crystallization. The steam does
not directly contact the ammonium sulfate solution, but transfers
heat indirectly thereto via a heat exchange unit. A
solvent-comprising vapor stream is formed in crystallization
section (1), and is discharged through line (7) to crystallization
section (2), where it is used to evaporate solvent, analogous to
the process in crystallization section (1). The solvent-comprising
vapor stream formed in crystallization section (2) is discharged
through line (8) to crystallization section (3) where it is used to
evaporate solvent analogous to the process in crystallization
section (1). The solvent-comprising vapor stream formed in
crystallization section (3) is discharged through line (9) to
crystallization section (4) where it is used to evaporate solvent
analogous to the process in crystallization section (1). The
solvent-comprising vapor stream formed in crystallization section
(4) is discharged via line (10). Ammonium sulfate crystals are
discharged from crystallization section (1) though line (11) for
further processing. A fraction of ammonium sulfate solution
comprising impurities is purged through line (12). Ammonium sulfate
crystals are discharged from crystallization section (2) though
line (13) for further processing. A fraction of ammonium sulfate
solution comprising impurities is purged through line (14).
[0080] Ammonium sulfate crystals are discharged from
crystallization section (3) though line (15) for further
processing. A fraction of ammonium sulfate solution comprising
impurities is purged through line (16). Ammonium sulfate crystals
are discharged from crystallization section (4) though line (17)
for further processing. A fraction of ammonium sulfate solution
comprising impurities is purged through line (18). Optionally, the
ammonium sulfate crystals from lines (11), (13), (15) and (17) are
combined, either before or after any further processing step. The
solutions of ammonium sulfate purged through lines (12), (14), (16)
and (18) are treated as waste, and undergo further processing.
Optionally, these purged solutions of ammonium sulfate are fed to
another crystallization section. Optionally, these solutions of
ammonium sulfate are combined.
[0081] FIG. 2 describes an embodiment of the present invention. The
system is essentially the same as that of FIG. 1. Specifically,
crystallization sections (1), (2), (3) and (4); the steam system
(6), (7), (8), (9), (10); the four lines through which ammonium
sulfate crystals are discharged from the crystallization sections
(11), (13), (15), (17); and purge lines (12), (14), (16) and (18)
are identical to those of FIG. 1.
[0082] The feeds of solutions of ammonium sulfate to
crystallization sections (1), (2), (3) and (4) are adapted. Instead
of feeding a common aqueous ammonium sulfate solution to each of
the crystallization sections (1), (2), (3) and (4), a first aqueous
ammonium sulfate solution that contains one or more impurities is
fed via line (5a) to a first group of crystallization sections,
comprising (1) and (2); and a second aqueous ammonium sulfate
solution that contains one or more impurities is fed via line (5b)
to a second group of crystallization sections, comprising (3) and
(4).
[0083] FIG. 3 describes an embodiment of the present invention. The
system is similar to that of FIG. 2 except that it comprises two
parallel series of four crystallizers, each series being heat
integrated. Specifically, crystallization sections (1), (2), (3)
and (4); the steam system (6), (7), (8), (9), (10); the four lines
through which ammonium sulfate crystals are discharged from these
crystallization sections (11), (13), (15), (17); and purge lines
(12), (14), (16) and (18) are identical to those of FIG. 2. A
parallel series of crystallization sections, (1a), (2a), (3a) and
(4a); steam system (6a), (7a), (8a), (9a), (10a); lines through
which ammonium sulfate crystals are discharged from these
crystallization sections (11a), (13a), (15a), (17a); and purge
lines (12a), (14a), (16a) and (18a) are analogous to the first
series of crystallization sections described with reference to FIG.
2. These correspond to the numbered components of FIG. 2 without
the `a`.
[0084] The feeds of aqueous ammonium sulfate solutions to the
crystallization sections are adapted. To crystallization sections
(1), (2), (1a), (2a) and (3a) a first aqueous ammonium sulfate
solution is fed via line (5e). To crystallization sections (3), (4)
and (4a) a second aqueous ammonium sulfate solution is fed via line
(50. To each crystallization section roughly a similar amount of
ammonium sulfate solution is fed. Accordingly, crystallization
sections (1) and (2) form a first group; crystallization sections
(3) and (4) form a second group; crystallization sections (1a),
(2a) and (3a) form a third group; and crystallization section (4a)
forms a fourth group.
[0085] The invention is illustrated by but not intended to be
limited to the following Examples.
EXAMPLE 1
[0086] In a commercial caprolactam plant cyclohexanone oxime was
produced according to the Raschig route from cyclohexanone produced
via hydrogenation of phenol. The cyclohexanone oxime was converted
into caprolactam in a multi-stage Beckmann rearrangement process
with oleum. The obtained caprolactam was recovered after
neutralization with aqueous ammonia. In each of the cyclohexanone
oxime formation step and caprolactam formation step, aqueous
ammonia was used for neutralization. As a result an aqueous
ammonium sulfate solution was obtained as by-product in each
step.
[0087] The composition of the aqueous ammonium sulfate solution
obtained in the cyclohexanone oxime formation step was:
TABLE-US-00001 Ammonium sulfate ca. 43.5 wt. % Water ca. 54.4 wt. %
Free H.sub.2SO.sub.4 <0.1 wt. % COD ca. 120 ppm Ammonium nitrate
ca. 2.1 wt. %
[0088] The composition of the aqueous ammonium sulfate solution
obtained in the caprolactam formation step was:
TABLE-US-00002 Ammonium sulfate ca. 44 wt. % Water ca. 56 wt. %
Free H.sub.2SO.sub.4 <0.1 wt. % COD 1200 ppm Ammonium nitrate
<0.01 wt. %
[0089] COD (chemical oxygen demand) content, which is a measure for
the concentration organic impurities, refers to values as
determined according to ASTM D 1252-95 (dichromate method).
[0090] The volume:volume ratio of aqueous ammonium sulfate solution
obtained in the cyclohexanone oxime formation step to the aqueous
ammonium sulfate solution obtained in the caprolactam formation
step was approximately 5:3.
[0091] By addition of aqueous ammonia (about 25 wt. %) the pH value
of both ammonium sulfate solutions were increased to about 5 (as
determined at a temperature of 25.degree. C.).
[0092] The resulting solutions were fed to two lines of each four
crystallization sections, in a system depicted in FIG. 3.
[0093] To crystallization sections (1), (2), (1a), (2a) and (3a)
the pH adjusted aqueous ammonium sulfate solution obtained in the
cyclohexanone oxime formation step was fed via line (5e). To
crystallization sections (3), (4) and (4a) the pH adjusted aqueous
ammonium sulfate solution obtained in the caprolactam formation
step was fed via line (5f). To each crystallization section roughly
a similar amount of ammonium sulfate solution was fed.
[0094] The crystallizers in the crystallization sections (1) and
(1a) were operated at a temperature of about 115.degree. C. The
crystallizers in the crystallization sections (2) and (2a) were
operated at a temperature of about 90.degree. C. The crystallizers
in the crystallization sections (3) and (3a) were operated at a
temperature of about 70.degree. C. And those in the sections (4)
and (4a) were operated at a temperature of about 50.degree. C. All
crystallizers were of the Oslo crystallizer type.
[0095] The amount of fresh steam that were fed via lines (6) and
(6a) to the crystallization sections (1) and (1a) was in each case
about 10 ton/hr.
[0096] By purging aqueous ammonium sulfate solution, COD levels in
the crystallization sections (4), (3a) and (4a) were kept at levels
of approximately 40, 30 and 40 gram per kg clear solution,
respectively. By purging aqueous ammonium sulfate solution, the
ammonium nitrate levels in clear solution in the crystallization
sections (1), (2), (1a), (2a) and (3a) were kept at levels of
approximately 35 wt. %. From each crystallization section, ammonium
sulfate solution containing ammonium sulfate crystals was
discharged and fed to a centrifuge in which the crystals were
separated from mother liquor and were washed with some water. Then
the obtained washed crystals were dried.
[0097] The colour of the resulting ammonium sulfate crystals was
white and no black coloured particles were observed between the
salt crystals.
[0098] The production capacity of ammonium sulfate crystals of each
crystallization section was about 60 kta.
[0099] This example shows that by feeding an aqueous ammonium
sulfate solution obtained in the cyclohexanone oxime formation step
to the crystallization sections that are operated at higher
temperatures and feeding an aqueous ammonium sulfate solution
obtained in the caprolactam formation step it is possible to
produce ammonium sulfate crystals that are not polluted with black
coloured particles.
[0100] The combined amount of fresh steam that was fed via lines
(6) and (6a) to the crystallization sections (1) and (1a) was about
20 ton/hr, In case both aqueous ammonium sulfate solutions would
have been fed to 8 crystallization sections without heat
integration the total consumption of fresh steam would have been
for each section about 10 ton/hr. So, this example further shows
that steam (energy) consumption may be significantly reduced; in
theory by 75%.
COMPARATIVE EXAMPLE 1
[0101] In a commercial caprolactam plant cyclohexanone oxime was
produced according to the Raschig route from cyclohexanone produced
via hydrogenation of phenol. The cyclohexanone oxime was converted
into caprolactam in a multi-stage Beckmann rearrangement process
with oleum. The obtained caprolactam was recovered after
neutralization with aqueous ammonia. In each of the cyclohexanone
oxime formation step and in the caprolactam formation step aqueous
ammonia was used for neutralization and as a result an aqueous
ammonium sulfate solution was obtained as by-product.
[0102] The volume:volume ratio of the amount of aqueous ammonium
sulfate solution obtained in the cyclohexanone oxime formation step
to the amount of aqueous ammonium sulfate solution obtained in the
caprolactam formation step was approximately 5:3. These two aqueous
ammonium sulfate solutions were blended.
[0103] The composition of the combined aqueous ammonium sulfate
solutions was:
TABLE-US-00003 Ammonium sulfate ca. 43.7 wt. % Water ca. 55 wt. %
Free H.sub.2SO.sub.4 <0.1 wt. % COD ca. 525 ppm Ammonium nitrate
ca. 1.3 wt. %
[0104] By addition of aqueous ammonia (about 25 wt. %) the pH value
of combined ammonium sulfate solutions was increased to about 5 (as
determined at a temperature of 25.degree. C.).
[0105] The obtained pH adjusted aqueous ammonium sulfate solution
was fed to all four crystallization sections of an experimental
set-up as described in FIG. 1. To each crystallization section
roughly a similar amount of ammonium sulfate solution was fed.
[0106] The temperatures of the crystallizers in the crystallization
sections (1), (2), (3) and (4) were about 115.degree. C.,
90.degree. C., 70.degree. C. and 50.degree. C., respectively.
[0107] In order to obtain the same overall ammonium sulfate crystal
yield as Example 1 the ratios of purge flow over feed for each
crystallizer were taken equal to those in Example 1. Specifically,
the ratio of purge flow over feed of crystallization section (1)
was taken equal to the average of the ratios of purge flow over
feed of crystallization sections (1) and (1a) in Example 1; the
ratio of purge flow over feed of crystallization section (2) was
taken equal to the average of the ratios of purge flow over feed of
crystallization sections (2) and (2a) in Example 1; the ratio of
purge flow over feed of crystallization section (3) was taken equal
to the average of the ratios of purge flow over feed of
crystallization sections (3) and (3a) in Example 1; and the ratio
of purge flow over feed of crystallization section (4) was taken
equal to the average of the ratios of purge flow over feed of
crystallization sections (4) and (4a) in Example 1.
[0108] From all crystallizers, the flows containing ammonium
sulfate crystals were discharged and via centrifugation the
crystals were separated from mother liquor and were washed with
water. Then the obtained washed crystals were dried.
[0109] The colour of the resulting ammonium sulfate crystals
obtained from crystallization sections (1) and (2) was brownish and
black coloured particles could be observed between the salt
crystals. In the ammonium sulfate crystals obtained from
crystallization sections (3) and (4) no black coloured particles
were observed.
[0110] This example shows that by feeding a blend of the aqueous
ammonium sulfate solution obtained in the cyclohexanone oxime
formation step and the aqueous ammonium sulfate solution obtained
in the caprolactam formation step it is possible to produce
ammonium sulfate crystals with the same overall ammonium sulfate
crystal yield per tonne of produced ammonium sulfate crystals due
to operating the crystallizers with the same purge to feed rates as
Example 1. It is clear that after implementing this heat
integration the same low overall consumption of heating steam per
tonne of produced ammonium sulfate crystals can be obtained as
Example 1.
[0111] However, due to the poor quality of the ammonium sulfate
crystals produced in the crystallization sections (1) and (2) the
average quality of all ammonium sulfate crystals produced in
Comparative Example 1 is much worse than the average quality of all
ammonium sulfate crystals produced in Example 1.
COMPARATIVE EXAMPLE 2
[0112] In this Comparative Example 2 the same blend of two aqueous
ammonium sulfate solutions was used as in Comparative Example 1. By
addition of aqueous ammonia (about 25 wt. %) the pH value of
combined ammonium sulfate solutions was increased to about 5 (as
determined at a temperature of 25.degree. C.).
[0113] The obtained pH adjusted aqueous ammonium sulfate solution
was fed to the crystallization sections (3) and (4) of an
experimental set-up as described in FIG. 1. The crystallization
sections (1) and (2) of the experimental set-up as described in
FIG. 1 were not in operation. Fresh steam was fed to the
crystallization section (3) via line (8). The temperatures of the
crystallizers in the sections (3) and (4) were taken equal to those
in Comparative Example 1: about 70.degree. C. and 50.degree. C.,
respectively.
[0114] The ratio of purge flow over feed of crystallization section
(3) was taken equal to the ratio of purge flow over feed of
crystallization section (3) in Comparative Example 1; and the ratio
of purge flow over feed of crystallization section (4) was taken
equal to the ratio of purge flow over feed of crystallization
section (4) in Comparative Example 1.
[0115] From all crystallizers ammonium sulfate solution containing
ammonium sulfate crystals was discharged and via centrifugation the
crystals were separated from mother liquor and were washed with
water. Then the obtained washed crystals were dried.
[0116] The resulting ammonium sulfate crystals obtained from
crystallization sections (3) and (4) were white coloured and no
black coloured particles were observed.
[0117] This example shows that by feeding a blend of the aqueous
ammonium sulfate solution obtained in the cyclohexanone oxime
formation step and the aqueous ammonium sulfate solution obtained
in the caprolactam formation step it is possible to produce good
quality ammonium sulfate crystals (white coloured and without black
particles).
[0118] However, operating the evaporative crystallizers heat
integrated in a series of just two instead of four results in an
overall consumption of heating steam per tonne of produced ammonium
sulfate crystals that is almost twice as high as the overall
consumption of heating steam per tonne of produced ammonium sulfate
crystals produced in Example 1.
COMPARATIVE EXAMPLE 3
[0119] In a commercial caprolactam plant caprolactam is produced
from cyclohexanone oxime via a 3-stage Beckmann rearrangement
process in oleum. The obtained caprolactam was recovered after
neutralization of the reaction mixture with aqueous ammonia. The
resulting aqueous ammonium containing sulfate solution was
extracted with benzene to recover caprolactam. After stripping, the
resulting aqueous ammonium sulfate containing solution was sent to
the crystallization section. Here the pH value of the stripped
aqueous ammonium containing sulfate solution which had a
temperature of about 60.degree. C. was adjusted by adding aqueous
ammonia to a value of about 5 (as determined at a temperature of
25.degree. C.). The resulting solution was fed to an Oslo type
crystallizer that was operated at a temperature of about
115.degree. C. The annual capacity of this Oslo crystallizer was
about 75 kton ammonium sulfate crystals. By purging ammonium
sulfate solution, the COD level in the crystallizer was kept at a
level of approximately 15 gram per kg clear solution. Ammonium
sulfate solution containing ammonium sulfate crystals was
discharged from this crystallizer and fed to a centrifuge in which
the crystals were separated from the mother liquor and washed with
water. Then the obtained washed crystals were dried.
The colour of the resulting ammonium sulfate crystals was brownish,
and black coloured particles that were irregularly shaped and of
sizes up to a few millimetres could be observed between the salt
crystals.
[0120] Four of these black coloured particles were hand-picked and
analysed. The results of these analyses are shown in the next
Table:
TABLE-US-00004 Component Particle 1 Particle 2 Particle 3 Particle
4 Water 12.3 wt. % 30.4 wt. % 1.4 wt. % 5.8 wt. % Ammonia 14.5 wt.
% 6.0 wt. % 9.7 wt. % 11.0 wt. % Sulfate 35.0 wt. % 8.6 wt. % 17.6
wt. % 21.2 wt. % Caprolactam 0.16 wt. % 0.14 wt. % 0.16 wt. % 0.5
wt. % .epsilon.-aminocaproic 0.28 wt. % 0.22 wt. % 0.43 wt. % 0.43
wt. % acid Disulfonated 9.9 wt. % 15.4 wt. % 19.8 wt. % 20.2 wt. %
octahydro- phenazine Others Balance Balance Balance Balance
Non-aqueous 3.2 wt. % 4.4 wt. % 6.9 wt. % 7.6 wt. % soluble
residue
[0121] This Comparative Example 3 shows that undesired visible
solid impurities are present when a feed of ammonium sulphate
formed as by-product in the Beckmann rearrangement of cyclohexanone
oxime to form caprolactam, when it is crystallized at a temperature
of 115.degree. C. Further, that these undesired visible solid
impurities have a high organic content.
* * * * *